Chemical Degradation of a Multiply Alkylated Cyclopentane (MAC) Oil During Wear: Implications for Spacecraft Attitude Control System Bearings

Abstract

The wear-induced chemical degradation process was examined using deliberately under-lubricated thrust ball bearings to create wear in a reasonable time frame. The liquid-phase products were analyzed using infrared spectroscopy and gel-permeation chromatography. Our results indicate that oil molecules react with the freshly created clean Fe surface produced during wear, and fragment into radicals by breaking C–C bonds. These free radicals produce alkenes, which can then be further attacked by radicals, leading to the formation of larger molecules (oligomers and eventually polymers). A side reaction, production of iron carboxylate salts, also occurred during wear. Additional experiments indicate that organic acids can be produced by radical reaction with the oxidized areas of the bearing surfaces and wear particles, and these acids can then attack the clean Fe surfaces created by wear to form iron carboxylate salts. We used our experimental results to estimate the amount of degradation likely to be produced in attitude control system bearing tests under conditions that produce wear and poor performance. We find that multiply alkylated cyclopentane (MAC) oil chemical degradation is an effect of wear, and is insufficient to be a cause of failure itself. Life predictions for MAC-lubricated attitude control system bearings must be based on predictions of oil transport and wear since failures are due to the failure of the lubrication system, not of the lubricant itself.